During total hip replacement surgery, placement of the acetabular cup is primordial. The accurate position of the acetabular cup is critical for good functioning of the implant resulting in successful clinical outcome. Malpositioning if the acetabular cup will lead to significant complications resulting in increased risk of dislocation, premature wear of the bearing surfaces and release of polyethylene debris and particles that will result in osteolysis and acetabular component loosening. Impingement is a major source of loosening and metallosis, where metallic debris lead to severe local tumor like inflammatory reactions. This will ultimately lead to extensive revisions of the implants.
Acetabular cup orientation is defined by the inclination and anteversion of the equatorial plane of said cup vis-à-vis the cardinal coronal and sagittal planes passing vertically through the center of the pelvis. Optimal orientation of the cup will prevent dislocation, increase range of motion and reduce edge loading and impingement. Postoperative radiographs measurements have shown that 50% of the cups inserted by experienced surgeons were outside the desired “safe zone” of Lewinneck, defined as a cup placed at 45 degrees of inclination and 15 degrees of anteversion
The problem with the conventional methods of orienting the acetabular cup during total hip surgery is that they rely on the position of patient during the surgical procedure notwithstanding the fact that the position of the pelvis while patient is covered under surgical drapes may not be accurately flat but rotated or tilted. Therefore, relying on the position of the patient may lead to significant inaccuracy of the final inclination and the anteversion of the implanted acetabular cup. At times, surgeons use inclination devises attached to cup inserter and pointed in the direction of the patient's shoulder. Again, the position of the patient under the surgical drapes may be rotated and not completely supine or flat leading to malposition of the implanted cup. More invasive techniques have been in use recently including computerized navigation. However, these devices require invasive insertion of probes into anatomical references such as the iliac crest and other anatomical pelvic landmarks such as the anterior iliac spines. The use of these probes requires separate skin incisions leading and subsequent local skin irritations. Furthermore, navigation devices require expensive monitoring devices and assigned and specially trained technicians to register the specifically designated landmarks. Said registration can be lengthy and time consuming requiring sophisticated monitoring devices and equipment.